In a sewing machine having two needles which are arranged such that eyes of the respective needles are at different heights from a throat plate, there has been known a threading device that is operable to thread each of the eyes of the two needles. Such a threading device may include a threading shaft which is disposed adjacent to a needle bar and having a threading hook at a lower end portion thereof, an turning base which is rotatably supported on a sewing machine frame via a support shaft disposed parallel to the needle bar and supporting the threading shaft so as to be movable in a vertical direction, a threading lever from which a downward moving operation is input to the threading shaft, and a threading guide which determines a height of the threading hook at the time of a threading operation (see, e.g., JP 10-137481 A).
The threading shaft is supported so as to be rotatable and vertically movable with respect to the turning base. The threading hook is disposed at the lower end portion of the threading shaft so as to extend in a circumferential direction around the threading shaft. The threading operation is implemented by rotating the threading shaft in one direction such that the threading hook is inserted through the eye of the one of the needles in order to catch a thread ahead thereof, and by rotating the threading shaft in a reverse direction such that the threading hook is removed out from the eye of the needle in order to pull in the thread through the eye.
The threading lever is formed with an oblique cam groove, while the threading shaft is provided with a follower pin engaging with the cam groove. The threading shaft is further provided with a guide pin which comes into contact with an abutting surface of the threading guide during a downward movement of the threading shaft. Accordingly, when the threading lever is pressed downward, the threading shaft is moved downward together with the threading lever, and then the guide pin of the threading shaft comes into contact with the abutting surface of the threading guide during the downward movement of the threading shaft, whereby the downward movement of the threading shaft becomes restricted. When the threading lever is further operated downward, the follower pin is relatively moved upward with respect to the cam groove so that the threading shaft is rotated with respect to the threading lever, whereby the threading hook is moved forward. At this time, the threading hook is inserted into the eye of either of the needles, and the threading work is carried out.
Meanwhile, the two needles are disposed at different positions, and heights of the eyes of the respective needles are different from each other. In the threading device, however, it is possible to cause the threading hook to pass through a position of each of the needles by changing a position of the threading shaft through a turning operation of the turning base. Furthermore, in order to deal with the difference of the heights between the eyes of the respective needles, the threading guide has two abutting surfaces having different heights so that the guide pin is caused to be in contact with either one of the abutting surfaces corresponding to the position of the threading shaft which has been changed by the turning operation of the turning base.
More specifically, when the threading lever is pressed downward with the turning base being adjusted to a certain angle, the threading shaft is moved downward together with the threading lever, and the guide pin comes into contact with one of the abutting surfaces, whereby the threading shaft is rotated by the interaction between the follower pin and the cam groove so that the threading hook is inserted into the eye of one of the needles to thread the corresponding needle. When the threading lever is pressed downward with the turning base being adjusted to another angle, because the arrangement of the threading shaft is switched, the guide pin comes into contact with the other abutting surface during the downward movement of the threading shaft through the operation of the threading lever, whereby the threading shaft is rotated so that the threading hook is inserted into the eye of the other needle to thread the corresponding needle.
Meanwhile, in recent years, some sewing machines are operable to implement a plurality of kinds of stitches. For example, in a sewing machine operable to implement both an overlock stitch and a cover stitch, five needles are provided. In such a sewing machine having three or more needles, an arrangement of the needles are sometimes complex not only in that heights of respective eyes of the needles are different, but also in that the needles are not arranged in a row, e.g., the needles are arranged so as to be separated in a cloth feeding direction. In such case, it has been difficult to guide the threading hook to each of the needles with the positional adjustment of the threading shaft through the turning operation of the turning base as in the sewing machine described above.